Microprocessor controlled defrost termination

ABSTRACT

An apparatus and method are disclosed for terminating a refrigeration unit&#39;s defrost function. The refrigeration unit comprises an evaporator, a temperature sensor to measure the temperature of the evaporator during a defrost function, and a controller configured to calculate the rate of temperature change and terminate the defrost function when the rate meets a specified criteria, such as a predetermined rate or a sharp increase in the rate after the evaporator temperature has increased above the freezing point of water.

CROSS-REFERENCE TO RELATED APPLICATION

Reference is made to, and this application claims priority from and thebenefit of U.S. Provisional Application Ser. No. 61/161,269, filed Mar.18, 2009, and entitled MICROPROCESSOR CONTROLLED DEFROST TERMINATION.

BACKGROUND OF THE INVENTION

This invention relates generally to refrigerated devices having cooledenclosures, and more specifically to detecting when an accumulation ofice on an evaporator associated with the refrigerated device has beenremoved during a defrost operation.

Refrigeration containers include refrigeration units for cooling. As iswell known in the art, a refrigeration unit has a compressor driven by acompressor motor, a condenser, a condenser fan driven by a condenser fanmotor, an evaporator, and an evaporator fan driven by an evaporator fanmotor. Refrigerant is circulated through the compressor, condenser, andevaporator, which are connected by refrigerant tubes. The operation of arefrigerator is controlled by a microprocessor or programmablecontroller. The controller is responsible for maintaining thetemperature within the enclosure by controlling the refrigeration unit.More specifically, the controller regulates run times of the compressormotor, condenser fan motor, and evaporator fan motor. The controller hasa time measurement device, or internal clock, to measure elapsed timefor a variety of conditions.

As the refrigeration unit operates, water vapor condenses on theevaporator. When the evaporator operates at temperatures below freezing,this water freezes on the evaporator, resulting in frost and icebuildup. The frost and ice buildup restricts air flowing through theevaporator, and the ability for heat transfer to occur between the airand the evaporator, which detracts from the refrigeration unit's coolingefficiency. To enhance the efficiency of refrigerators, defrostfunctions are instituted, whereby the ice or frost buildup is thawed andremoved.

These defrost functions typically occur periodically, oftenautomatically when ice or frost buildup on the evaporator is detected.When the defrost function starts, the cooling process stops and theevaporator is heated rather than cooled, thereby melting the frost andice. This heating can be accomplished by reversing the refrigerationcycle (referred to as reverse cycle defrost). Additionally, a resistiveheating element can be used to assist heating the evaporator (referredto as electric defrost). In any case, the refrigeration function ceases.Running the defrost function is necessary to improve the efficiency ofrefrigeration. However, the defrost function consumes a lot of energysince the unit is heated during this time rather than cooled. Currently,typical defrost functions run until the evaporator reaches a specifiedtemperature often well above the point at which all the frost or ice hasbeen removed. Alternative defrost functions use a pressure sensor orpressure switch. Some run for a predetermined amount of time. All thesefunctions heat the refrigerator, and hence, any items in therefrigerator for a period of time longer than necessary to fully defrostthe evaporator. This effective reduction in cooling time wastes energyand increases the instability of the refrigeration container'stemperature.

It would be advantageous to save energy and produce more stable,constant refrigeration temperatures by terminating the defrost functiondynamically, dependent on and closer to the point in time at which iceis fully removed from the evaporator.

SUMMARY OF THE INVENTION

In one embodiment of the invention, a microprocessor controlledrefrigeration unit is provided that terminates a defrost function basedon the point in time at which ice or frost buildup is removed from anevaporator component of the refrigeration unit. A temperature sensor isprovided to measure the temperature of the evaporator. A microprocessoris provided capable of calculating rates of temperature change in theevaporator during the defrost function, and terminating the defrostfunction when the rate of temperature change meets a predeterminedcondition or criteria.

In another embodiment of the invention, a method is provided toterminate the defrost function in a refrigeration unit based on thepoint in time at which ice or frost buildup is removed from anevaporator component of the refrigeration unit. During the defrostfunction, the rate of temperature increase is measured or calculated.When the rate of temperature change meets a predetermined condition, thedefrost function is terminated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a mechanical block diagram according to one embodiment of theinvention.

FIG. 2 is an electrical block diagram of a refrigeration containeraccording to one embodiment of the invention.

FIG. 3 is a flow chart depicting operation of a defrost functiontermination scheme according to one embodiment of the invention.

FIG. 4 is a graphical representation showing a basis for the defrostfunction termination according to one embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

In the detailed description that follows, identical components have beengiven the same reference numerals, and in order to clearly and conciselyillustrate the present invention, certain features may be shown insomewhat schematic form.

FIGS. 1 and 2 illustrate a refrigeration unit 10 for cooling a containeror device. Because refrigeration systems are well known, and theinvention can be adapted to work with many, if not all conventionalrefrigeration units, FIGS. 1 and 2 are highly schematic. One skilled inthe art will appreciate that the invention can be adapted for use inmany refrigerated devices, such as, but not limited to, commercialrefrigerator/freezer combinations, commercial stand-alone freezers,residential refrigerator/freezers, and transportable refrigerationcontainers.

Referring to FIGS. 1 and 2, the refrigeration unit 10 has a compressor12 driven by a compressor motor 14, a condenser 16, a condenser fan 18driven by a condenser fan motor 20, an evaporator 22 and an evaporatorfan 24 driven by an evaporator fan motor 26. The motors 14, 20, and 26can be powered by a power source 34. An optional defrost heater 38 canalso be powered by the power source 34. Refrigerant is circulatedthrough the compressor 12, condenser 16, and evaporator 22, which areconnected by tubes 28. The operation of the refrigerator 10 iscontrolled by a processor or programmable controller 30. By controllingpower through relays 36, the controller 30 regulates when the compressormotor 12, condenser fan motor 20, evaporator fan motor 26, and optionaldefrost heater 38 operate.

During refrigeration, water vapor condenses on the evaporator 22 at anytime the evaporator temperature is below the dew point of the airpassing through. When the evaporator temperature is below the freezingpoint of water, the condensation on it can freeze, resulting in frost orice buildup on the evaporator 22. This frost or ice buildup obscures theevaporator 22 and blocks its surrounding air space, causing a lessefficient refrigeration process. The controller 30, through variousmechanisms known in the art, periodically or as necessary, initiates adefrost function to remove any frost or ice buildup on the evaporator22. The defrost function entails stopping the cooling operation of therefrigeration unit 10. Typically, during defrost, the refrigeration unitruns in reverse in order to heat the evaporator 22 and melt any frost orice. Sometimes, a resistive heater 38 is used alone or in combinationwith the above-described method to defrost the evaporator 22. The term“defrost means” will be used to mean any combination of the abovedescribed apparatus and methods of defrosting, as well as any otherapparatus and methods of defrosting.

The controller 30 has a time measurement device, or internal clock, tomeasure elapsed time. In one embodiment of the invention, a temperaturesensor 32 is able to record the surface temperature of the evaporator 22over continuous intervals. The temperature readings can be converted toelectrical signals and electrically communicated to the controller 30.The controller 30, or another processor, is configured to calculate therate of temperature change in the evaporator 22 using the temperaturemeasured over time. Although FIG. 1 schematically depicts onetemperature sensor 32, multiple temperature sensors 32 can be used.

Depending on the particular refrigerator, these sensors 32 can be placedon the structural support or the refrigerant tubes of the evaporator 22,as ice can collect in both places. For example, in one embodiment usinga reverse cycle defrost, it can be preferable to attach sensor(s) 32 tothe structural support of the evaporator 22 where ice will melt lastbecause heating occurs from the fluid in the refrigerant tubes. Inanother embodiment, such as one with an electric defrost, it can bepreferable to attach sensor(s) 32 to the refrigerant tubing or thestructural support, or both. Lastly, it is also conceived to mount thesensors variously, so that the refrigerant inside the evaporator 22 canbe measured, or the air passing through the evaporator can be measured.One skilled in the art will appreciate various locations or methods bywhich the temperature sensor(s) 32 can be mounted to measure thetemperature inside the evaporator 22.

Referring additionally to FIG. 3, the operation of the refrigerationunit, with respect to the termination of the defrost function, isdescribed. During the defrost function, the controller 30 monitors thetemperature of the evaporator 22. The temperature sensor 32, measuresthe temperature of the evaporator 22, according to box 102, and providesthe temperature to the controller 30. Temperature measurement does notnecessarily need to be direct. Another physical characteristic of theevaporator 22 can be directly measured that can be related totemperature and used to indicate when the evaporator 22 has reachedapproximately the freezing point of water. For instance, the pressureinside the evaporator 22 can also be measured and used to indicate thetemperature of the evaporator 22. Measuring another physicalcharacteristic of the evaporator 22 as a proxy for temperature isconsidered to be “measuring the temperature” as stated herein.

When the temperature reaches approximately the freezing point of water,according to decision box 104, the controller 30 begins calculating therate of temperature change, according to step 106. This rate can becalculated prior to this point, but proceeding to step 110 requires thetemperature of the evaporator 22 to have reached approximately thefreezing point of water. Furthermore, the measured temperature need notnecessarily be directly compared to determine if the evaporator 22 hasreached approximately the freezing point of water. This determinationcan be made in other ways. For instance, the decrease in positivetemperature change rate that occurs in the evaporator 22 atapproximately the freezing point of water can be used to determine whenthe evaporator 22 has reached approximately the freezing point of water.This concept is explained below with regard to FIG. 4. In accordancewith decision box 108, the controller 30 continues to receivetemperature readings and calculate the rate of temperature change untilthe rate meets a predetermined condition or criteria. The condition canbe programmed into the controller 30. Once the condition has been met,the controller terminates the defrost function, box 110 of FIG. 3, byrestoring normal operation of the refrigeration unit 10. By this mannerof terminating the defrost function, the termination temperature floats.Rather than terminating based on a predetermined temperature of thecoil, or a predetermine length of time, termination is dependent on theactual point in time ice is melted.

The schematic graphical depiction of FIG. 4 illustrates the principlebehind predetermining the condition. The condition is based on qualitiesregarding the rate at which the temperature of the evaporator 22 riseswhile and after ice and frost melts off the evaporator 22. Duringrefrigeration, the evaporator 22 operates well below the freezing pointof water. During defrosting, when ice exists on an evaporator 22, theevaporator 22 is heated toward the freezing point of water. When theevaporator 22 reaches the freezing point of water, if ice is stillpresent, the positive rate at which the evaporator temperature risesthen reduces. When ice is present on the evaporator 22, the rate changecan be abrupt. This significant event can be used to mark a point intime where the evaporator reaches approximately the freezing point ofwater. The rate remains reduced until most or all of the ice and frostmelts. Because of this rate change, then, in addition to marking theactual temperature of the evaporator 22, marking the rate decrease orthe difference between the rates at slope segments 200 and 210 can beused to determine the point in time when the temperature reaches thefreezing point of water.

The rate change is significant, for instance, if it can be identifiedand distinguished. The characteristics of the rate change can varydepending on the configuration of the system, particularly as theconfiguration relates to the thermal transfer qualities of the system.For instance, using a higher powered resistive heater 38 can speed themelting rate and affect the noticeable change in rate as the evaporator22 reaches the freezing point of water. Or in another instance, thesteadiness in rate of temperature increase before and after it pauses atthe temperature of the ice can vary according to the systemconfiguration. Therefore, the rate change is significant if it can beidentified, and in particular, if it can be distinguished from anynormal fluctuation in the steady rate. One skilled in the art willrecognize ways to identify and distinguish the rate change.

After the evaporator temperature approaches the temperature of ice onthe evaporator, the rate remains low until most or all of the ice melts.FIG. 4 reflects this significant pause in temperature change by theflatness of the curve over a period of time. Again, the pause can vary,this time depending further on how much ice is on the evaporator. Thepause is significant in that it is detectable and distinguishable. Atthe end of the pause, when the ice and frost has mostly or fully melted,the temperature increase resumes. There occurs a sharp increase in therate of temperature change. This is the point in time at which thedefrost function will be terminated. Similar to the decline when theevaporator temperature approaches the freezing point of water, theincrease will be significant.

This principle can be used to predetermine the condition upon which thecontroller relies to terminate the defrost function. With regard topredetermining the termination condition, in one embodiment, the valueto which the rate increases after the ice fully melts is predeterminedand programmed into the controller 30. When the measured rate reaches orexceeds the predetermined rate, the defrost function is terminated. Inanother embodiment, a minimum acceleration in temperature change rate isprogrammed into the controller. When that minimum acceleration is met,the controller terminates the defrost function. In yet anotherembodiment, the pause in temperature rise is detected and used toterminate the defrost function. For instance, the predeterminedcondition can be the detection of a pause or disruption in the rate fora length of time. In another embodiment, the difference between therates represented by the slope segments 210 and 220 is used to determinewhen to terminate the defrost function. Other alternatives relying onthe rate at which temperature changes, as depicted in FIG. 4, areconceived that one skilled in the art would recognize to be equivalentand within the scope of the invention.

In an alternate situation, if the evaporator 22 reaches the freezingpoint of water when heated during the defrost function, and little or noice is present (i.e. it has been entirely or almost entirely meltedalready) then there may be no change, little change, an insignificantchange, or a very brief change in the rate at which the temperature ofthe evaporator rises. The transition from slope segment 200 through 230to 240 depicts an instance in which very little ice is present when theevaporator temperature approaches and exceeds the freezing point ofwater. The slope adjusts for a short period of time. The transition fromslope segment 200 to 250 depicts an instance in which no ice is present.There is no change or almost no change in the rate of temperatureincrease. In the case where the rate change is insignificant,undetectable, or not meaningful, then the measured temperature of theevaporator 22 can still be used to determine the evaporator 22 hasreached the freezing point of water. Then, the predetermined conditionthat the rate of temperature rise would have to meet to signal thecontroller 30 to terminate the defrost function would be the absence ofany significant or detectable change after the evaporator reachedapproximately the freezing point of water.

Alternative embodiments of the refrigeration unit exist, as well,consistent with the scope of the invention that would be recognized bythose skilled in the art. For instance, one such embodiment would be theincorporation of the above-disclosed defrost function termination basedon rate of temperature change with a time-sensitive termination feature.That is, as a safety mechanism to prevent the chance of a prolongedheating of the refrigeration unit, the controller can be programmed toterminate the defrost function if it exceeds a certain time limit, or ifthe evaporator 22 exceeds a certain temperature or pressure. Otherexamples include the addition of fail safes, known in the art, to ensureoperation of the refrigeration unit and defrost function if one or morecomponents or features, such as the temperature sensor(s), fail to work.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to make and use the invention. The patentable scope of the inventionis defined by the claims, and may include other examples that occur tothose skilled in the art. Such other examples are intended to be withinthe scope of the claims if they have structural elements that do notdiffer from the literal language of the claims, or if they includeequivalent structural elements with insubstantial differences from theliteral language of the claims.

What is claimed is:
 1. A refrigeration unit comprising: an evaporator;defrost means for defrosting said evaporator; at least one temperaturesensor in measurable connection with said evaporator; and at least onecontroller communicatively connected to said at least one temperaturesensor, wherein said at least one controller is configured to calculatea rate of temperature rise and to terminate operation of said defrostmeans when said rate exhibits a predetermined criteria.
 2. Therefrigeration unit of claim 1, wherein said predetermined criteria is apredetermined rate.
 3. The refrigeration unit of claim 1, wherein saidpredetermined criteria is a significant increase.
 4. The refrigerationunit of claim 1, wherein said at least one controller is furtherconfigured to determine a point in time at which said temperature hasrisen to a freezing point of water.
 5. The refrigeration unit of claim4, wherein said predetermined criteria is a predetermined rate aftersaid point in time.
 6. The refrigeration unit of claim 4, wherein saidpredetermined criteria is a significant increase after said point intime.
 7. The refrigeration unit of claim 4, wherein said predeterminedcriteria is a lack of significant increase after a specified length oftime starting at said point in time.
 8. The refrigeration unit of claim1, wherein said predetermined criteria is a significant increase after asignificant decrease.
 9. A method for terminating a refrigerator defrostfunction, the method comprising: initiating a function to defrost anevaporator in a refrigeration unit; measuring the temperature of saidevaporator during said function to defrost said evaporator; calculatinga rate of temperature change of said evaporator, said calculation basedon said measured temperature; and terminating said function to defrostwhen said rate of temperature change exhibits a specified criteria. 10.The method of claim 9, further comprising the step of determining afirst point in time at which said temperature of said evaporator reachesapproximately the freezing temperature of water.
 11. The method of claim10, wherein said point in time is determined by a significant decline insaid rate of temperature change
 12. The method of claim 9, wherein saidspecified criteria is a predetermined value.
 13. The method of claim 9,wherein said specified criteria is a significant increase.
 14. Themethod of claim 10, wherein said specified criteria is a lack ofsignificant increase after a specified duration of time starting at saidpoint in time.
 15. The method of claim 9, wherein said step of measuringoccurs at continuous intervals over a length of time.
 16. The method ofclaim 9, wherein said step of calculating occurs at continuous intervalsover a length of time.
 17. A method for terminating a refrigeratordefrost function, the method comprising: operating a defrost function ina refrigeration unit; measuring the rate of temperature increase of anevaporator during said defrost function; and terminating said defrostfunction based on said rate of temperature change.
 18. The method ofclaim 17, the terminating step occurring when a significant increase isdetected in said rate of temperature change.
 19. The method of claim 17,said terminating step occurring when a said rate of temperature changeexceeds a predetermined rate value.